Method of controlling the strainsensitivity of bessemer steel



ing with the iron maytemperature atwhich the heat of Bessemer steel is LPatented July 6,

2,323,695 v METHOD OF CONTROLLING THE STRAIN;

SENSITIVITY F Richard R. Webster, Beaver, Pa., & Laughlin SteelCorporation,

BESSEMEB STEEL assignor to Jones Pittsburgh, Pa.,

a corporation of Pennsylvania Application Aprill, 1941, Serial No.386,260

Claims.

This invention relates in general to the manufacture of Bessemer steeland, in particular, to the control of the strain-sensitivity thereof.

All steel exhibits a change in physical properties when subjected tocold working, i. e., deformation at a tempperature below therecrystallization temperature. Thechange in physical properties isroughly proportional to the extent ofthe cold working but somesteel'qexhibits a greater change than others for the same degree of coldworking. Theterm strain-sensicold working.

vThe Graham et a]. Patent No. 2,174,740 discloses a method ofcontrolling the strain-sensi-. tivity ofsteel by adjustingthe'proportion of oppositely effected nitrides contained .therein. Ironnitrides increase the strain-sensitivity and aluminum nitridedecrea'sesit. In the Bessemer process of making. steel, EXCCWIOI additions priorto pouring, the blown heat is substantially free from thosenitrides'such as aluminum nitride which reduces the, strain-sensitivity.It is desirable, for certain purposes, to produce rimmed steel of lowstrain-sensitivity,but this cannot be produced by aluminum additions,because the addition of aluminum to reduce the strain-sensitivity kills"the steel and thereby prevents rimming thereof.

I have discovered that I can effect a control of the strain-sensitivityimilar to that obtained according to the practice of the Graham et al.

patent with additions trides, by controlling of iron or aluminum nithetemperature at which a heat of iron is blown in the Bessemer converter.Even though the permissible range of .008 percent or- .007 percent. Ifthe steel is blown at a higher temperature, e. g., 3040 F.,'

' tivity designates the index of the rate of change of physicalproperties for a given amount-of tion of the iron charged into theconverter.

the temperature of the metal during blowing is relatively narrow, I amable to obtain a considerable range of strain sensitivities in thefinished steel. By my method of controlling the strain-sensitivity ofBessemer steel, I am able to produce rimmed steel having lowstrain-sensitivity as well as killed steel of like characteristics.

Ina preferred practice of the invention, I control-the amount ofnitrides originally formed in the steel by the combination of thenitrogen .in the air blast with the iron from which thesteel isconverted. I have discovered that, in

s'teel'containing'n'o aluminum nitride, or similarly efiectivenitrides,the strain sensitivity depends on the nitrogen contentand thatthe amolmt of nitrogen-from the air blast combinbe controlled withinlimits by varyin the metal is blown. If a blown at a relatively lowtemperature, such as 2925 F., the nitrogen content in the finishedsteel' (in the form of iron nitride) is as low as the converter may thenitrogen content may perature of the melt is duced by charging scrap orore into the concent.

normal Bessemer practice and control the temperature of the melt inorder to obtain the desired nitrogen content and photo-cell to theradiation through the tuyresv and record the current produced thereby.

- Various methods of regulating the temperature of the melt during theblow in a Bessemer converter are known to the art. If the temtoo high,it may be resteam to-the air blast.

If thetemperature of the bath is too low, it may be' raised byside-blowing, i. e., by tilting the converter so that some of the tuyresare uncovered. F A

The temperature of the melt may also be controlled by varying thetemperature or composiverter or by admitting the iron has a high siliconcon'tentyit will be heated to ahigher-temperature during the blowing, bythe oxidation of the excesssilicon, than if the silicon content is low.Ferro silicon may even be added to the charge to increase thetemperature rise of the melt during the blow. The temperature of themelt issimilarly affected by the amount of carbon present in the iron.The temperature at which the iron is charged into be varied somewhat andnaturally has a direct efiect on the temperature. of the melt during theblow.

' My invention is useful in the manufacture of steel by the duplexprocess, 1. e., that in which the steel is first partially blown in a.Bessemer Figure 1 shows the variation of radiant-energy" emission duringthe entire course of the blow or a melt whose temperature was Figure 2is a similar curvefor the blow of a bcas high as .022 per-' Inpracticing my invention, I depart from the strain-sensitivity indeterrxiii red by other means, such as an optical pyrometer, to

be about 2990 F.;

melt whose temperature was about 3090" F. and

Figure 3 is a set of curves showing the strainsensitivity of two typicalsteels produced by my invention. I

In conventional Bessemer practice, the blower usually attempts tomaintain the melt at a temperature intermediate the permissibleextremes, e, g., about 2970 F., with a preference for a somewhat highertemperature, in order to avoid any possibility of skulling in theconverter. In practicing my invention. I blow the melt at a temperatureconsiderably higher than 2970 F., e. g., as high as 3050 F., or at atemperature considerably lower than the 2970" F., e. g., 2930 F.,depending on whether I desire to produce steel having high or lowstrain-sensitivity. Thus I deliberately approach either the temperatureat which damage to the converter lining occurs or that at which"skulling" occurs in the converter, contrary to the usual practice. inorder to obtain a product having the desired characteristics. Thetemperatures stated are those existing toward the end of the blow. Inthe early stages of the blow, i. e., prior to the removal of asubstantial amount of carbon, the combination of nitrogen with iron isslight at any blowing temperature.

After the blow has proceeded to the point at which the desired removalof carbon has been obtained (see Work Patent No. 2,207,309), thetemperature of the melt having been brought to the proper value towardthe end of the blow by the known methods of control previouslymentioned, the steel is poured from the converter in the known manner.in an aluminum-free condition and without the addition of anysubstantial amount of any other agent combining with nitrogen. Afterpouring, the steel is converted to usable form by the usual hot and coldworking procedures. a

Figure 2 shows the variation in the toughness (in resistance to the Izodimpact test) of two steels of different nitrogen contents when sub-.Iected to increasing amounts of cold working. In other words, Figure 2illustrates the relationship between the embrittlement of the steel andthe' degree of cold work to which it is subjected. These curves areobtained inthe manner disclosed in the Graham et al. patent abovementioned. Curve A in Figure 2 is characteristic of Bessemer steel madeaccording to my invention having anitrogen content of about .008percent. Curve B illustrates the strain-sensitivity characteristics ofsteel made according to my invention having a nitrogen content of about.020 percent.

It will be apparent from the foregoing description and explanation thatthe invention is characterized by important advantages over previoussteel-making practices. In the first place, the invention makes itpossible to produce rimmed Bessemer steel having a relatively lowstrainsensitivity. The invention may be applied, furtherinore, to themanufacture of killed steels which have been deoxidized-with silicon ormanganese but not aluminum.

Previous to my invention. there was no method known for controlling theabsorption of nitrogen by the steel during the blow in a Bessemerconverter. In ordinary Bessemer practice, variations in nitrogencontents from blow to blow .of 50% or more are common with consequentvariations in strain-sensitivity. This uncontrolled variation has beenlargely responsible for the view that Bessemer steel is unreliable oreven treacherous. By my invention, however, I am able to produceBessemer steel which is uniform from blow to blow in respect to itsstrain-sensitivity. As shown by the figures given above, the nitrogencontent of the steel may be rather closely controlled, despite the factthat the permissible range of temperatures in blowing Bessemer steel isonly from about 2900 F., at which temperature the full blown steel willbegin to solidify, and the maximum of about 3150 F. to which therefractories lining the converter may safely be subjected. The figuresalready given furnish a good example of the degree of control over thenitrogen content which I am enabled to achieve. The nitrogen content ofBessemer steel blown at 2930 F. is about .008 percent, while steel blownat a temperature of 3030 F. or above will have a nitrogen content ofabout .020 percent, 1. e., 2.5 times the nitrogen content of steel blownat a temperature only lower.

Although I have disclosed but a preferred practice of the invention, itwill be apparent that changes in the procedure described may be madewithout departing from the spirit of the invention or the scope of theappended'claims.

I claim:

1. In a method of controlling the strain-sensitivity of Bessemer steel,the steps including placing a heat of molten iron in a converter,blowing air therethrough, and adding scrap or ore to the heat during theblow to hold the temperature of the bath below about 2950 F., therebyproducing a controlled nitrogen content in the steel at the conclusionof the blow.

2. In a method of controlling the strain-sensitivity of Bessemer steel,the steps including placing a heat of molten iron in a converter,blowing air therethrough, and introducing a cooling agent into the heatduring the blow in an amount suilicient to hold the temperature of thebath below about 2950 F., thereby producing a controlled nitrogencontent in the steel at the conclusion oi the blow.

3. In a method of making Bessemer steel having a relatively lowstrain-sensitivity, the steps taining the temperature of the iron below2950' F. during the blow so that the amount of the.

nitrogen of the air blast combining with the iron is below .015%.

5. In a method of controlling the strain-sensitivity of Bessemer steel.the steps including placing a heat of molten iron in a converter,blowing air therethrough, maintainingthe iron substantially free fromaluminum, introducing a cooling agent into the heat during the blow andthereby maintaining the temperature of the iron below 2950 F. during theblow so that the amount of the nitrogen of the air blast combining withthe iron is below .015%.

. RICHARD R. WEBSTER.

